It was the managers fault. i remember Sarri wanting to keep Willian and Alonso.
That is why it's great that we have a second season with the same coach. it always help when we have 2 or 3 years of stability. So Lampard had one full season to assess the squad, something that never happen, because when Conte came in by the time he realize a player was crap he was out. Likewise Sarri.
Not only that but also having Cech helped a lot as wel.
And we also benefited because of market conditions.